This blog is where I post up various half-baked ideas that I have.

Recent Posts

27 June 2022

Many modes: a GATs pattern

As some of you may know, on May 4th Jack Huey opened a PR to stabilize an initial version of generic associated types. The current version is at best an MVP: the compiler support is limited, resulting in unnecessary errors, and the syntax is limited, making code that uses GATs much more verbose than I’d like. Nonetheless, I’m super excited, since GATs unlock a lot of interesting use cases, and we can continue to smooth out the rough edges over time. However, folks on the thread have raised some strong concerns about GAT stabilization, including asking whether GATs are worth including in the language at all. The fear is that they make Rust the language too complex, and that it would be better to just use them as an internal building block for other, more accessible features (like async functions and [return position impl trait in traits][RPITIT]). In response to this concern, a number of people have posted about how they are using GATs. I recently took some time to deep dive into these comments and to write about some of the patterns that I found there, including a pattern I am calling the “many modes” pattern, which comes from the chumsky parser combinator library. I posted about this pattern on the thread, but I thought I would cross-post my write-up here to the blog as well, because I think it’s of general interest.

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15 June 2022

What it feels like when Rust saves your bacon

You’ve probably heard that the Rust type checker can be a great “co-pilot”, helping you to avoid subtle bugs that would have been a royal pain in the !@#!$! to debug. This is truly awesome! But what you may not realize is how it feels in the moment when this happens. The answer typically is: really, really frustrating! Usually, you are trying to get some code to compile and you find you just can’t do it.

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13 June 2022

Async cancellation: a case study of pub-sub in mini-redis

Lately I’ve been diving deep into tokio’s mini-redis example. The mini-redis example is a great one to look at because it’s a realistic piece of quality async Rust code that is both self-contained and very well documented. Digging into mini-redis, I found that it exemplifies the best and worst of async Rust. On the one hand, the code itself is clean, efficient, and high-level. On the other hand, it relies on a number of subtle async conventions that can easily be done wrong – worse, if you do them wrong, you won’t get a compilation error, and your code will “mostly work”, breaking only in unpredictable timing conditions that are unlikely to occur in unit tests. Just the kind of thing Rust tries to avoid! This isn’t the fault of mini-redis – to my knowledge, there aren’t great alterantive patterns available in async Rust today (I go through some of the alternatives in this post, and their downsides).

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17 April 2022

Coherence and crate-level where-clauses

Rust has been wrestling with coherence more-or-less since we added methods; our current rule, the “orphan rule”, is safe but overly strict. Roughly speaking, the rule says that one can only implement foreign traits (that is, traits defined by one of your dependencies) for local types (that is, types that you define). The goal of this rule was to help foster the crates.io ecosystem — we wanted to ensure that you could grab any two crates and use them together, without worrying that they might define incompatible impls that can’t be combined. The rule has served us well in that respect, but over time we’ve seen that it can also have a kind of chilling effect, unintentionally working against successful composition of crates in the ecosystem. For this reason, I’ve come to believe that we will have to weaken the orphan rule. The purpose of this post is to write out some preliminary exploration of ways that we might do that.

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12 April 2022

Implied bounds and perfect derive

There are two ergonomic features that have been discussed for quite some time in Rust land: perfect derive and expanded implied bounds. Until recently, we were a bit stuck on the best way to implement them. Recently though I’ve been working on a new formulation of the Rust trait checker that gives us a bunch of new capabilities — among them, it resolved a soundness formulation that would have prevented these two features from being combined. I’m not going to describe my fix in detail in this post, though; instead, I want to ask a different question. Now that we can implement these features, should we?

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29 March 2022

dyn*: can we make dyn sized?

Last Friday, tmandry, cramertj, and I had an exciting conversation. We were talking about the design for combining async functions in traits with dyn Trait that tmandry and I had presented to the lang team on Friday. cramertj had an insightful twist to offer on that design, and I want to talk about it here. Keep in mind that this is a piece of “hot off the presses”, in-progress design and hence may easily go nowhere – but at the same time, I’m pretty excited about it. If it works out, it could go a long way towards making dyn Trait user-friendly and accessible in Rust, which I think would be a big deal.

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